I am getting a C4 (157 tooth bell, case fill, fmVB) I don't know what size converter to go with. Bigger the better? I want it to be lockup with ballooning plates if that makes a difference. My engine is in the ballpark of 320hp if that makes any difference.
You want the 9" converter. There is not enough room for the big bellhousing to fit the bigger converter. As far as stall speed goes we'd have to know what kind of cam you have.
Both of my cars have 164 tooth flexplates/bellhousings, etc, on their C4's. Note that the flexplate/converter bolt pattern is different between the two, and there are probably small bolt circle large converters, and vice versa.... Measure before you buy....Been there done that. A 10 in converter should fit fine in a 157 tooth bellhousing, as I think stock was 11 in or so.
I have a 10 in TCI in the 82 that works fine. It's about a 2800-3000 stall. Normally you don't use a lockup converter with a C4, that would be from a C5, which is another set of matchup deals.
Depending on your engine combo and planned rpm, an higher stall 10 in or even a 9 in would work. The 9 in is usually a race only 3500 and up stall unit.